Although polymers and adhesives having good characteristics can be--and traditionally have been--prepared in both organic solvent and aqueous emulsion systems, with the advent of stricter environmental and work place protection laws and the increasing pressure for the conservation of materials and energy more desirable alternate systems pose financial and ecological advantages which are of real value. In some instances, identifying a substituting alternate system which gives acceptable results poses a technical challenge which cannot easily be overcome.
As a separate but related issue, polymers and adhesives are frequently subjected to formulational refinements designed to enhance one property or another. The desired characteristics of an adhesive or polymer are therefore often tailored by way of control of stoichiometry, and in particular by way of control of extent of crosslinking. Although polymer stoichiometry and extent of crosslinking are well understood generally, the characteristics which such stoichiometry or crosslinking will ultimately yield cannot be predicted with any certainty.
Notwithstanding the above, adhesives are known in which stoichiometry or extent of crosslinking are identified as responsible for one or more properties of the adhesive. U.S. Pat. No. 4,623,593 to Baier et al. discloses a self-adhesive polymer composition for use as a prosthetic appliance. The patent teaches that the pressure-sensitive adhesive properties of the disclosed composition result from the absence of complete crosslinking in the surface layer of the pressure-sensitive adhesive areas.
U.S. Pat. No. 3,740,366 to Sanderson et al. discloses an acrylic pressure-sensitive adhesive in which some of the carboxylic acid groups present are bound or crosslinked by a polyvalent metal. For example, a pressure-sensitive adhesive is prepared from a polymer containing free carboxylic acid groups derived from up to 4.5 percent of the total monomers of an alpha, beta ethylenically unsaturated carboxylic acid, by including in an aqueous dispersion or solution of the polymer a compatible polyvalent metal compound. Ionic crosslinking of carboxylate groups occurs upon removal of the aqueous media, at least theoretically, with no appreciable loss of tackiness in the adhesive, while the polyvalent metal/carboxylic acid bonds are believed to contribute to the improved shear resistance which such polymeric adhesives demonstrate. Sanderson et al. contemplate the use of solvent systems, although emulsion polymerization (presumably aqueous) systems are preferred.
Improved shear resistance is a common goal of adhesives design. Otherwise satisfactory adhesives, that is, adhesives which demonstrate good cohesive and adhesive strength, often demonstrate poor shear resistance. The reasons for this are not well understood, but improvement in shear resistance continues to be a goal in adhesive development. An ideal adhesive, therefore, would be manufactured from a non-solvent system, would demonstrate superior cohesive and adhesive strength, and would demonstrate shear resistance comparable to that of adhesives prepared from solvent systems or of adhesives containing expensive additives to which improved shear resistance is attributed.